Furburt I'm surprised at you!
I love MGS. Definitely my favourite game franchise. So obviously yes OP you should buy them. That said, here are some observations, probably presented as facts though they are really just my opinions:
MGS1 is the best game of the series. Not the TERRIBLE GameCube remake, but the original and groundbreaking game on the Playstation. This is the one you should play if you want to really understand the series.
If you enjoy MGS1, play MGS2. The gameplay is significantly improved, but I came away from that game more disappointed than I was after I discovered that there were no sequels to The Matrix (NO SEQUELS AND I WILL NOT HEAR OTHERWISE) because the plot is so shockingly and painfully bad. Still a reasonably enjoyable game.
Regardless of how much you enjoy those two, definitely play MGS3. It's the most complete one of the series and I suspect that if it had come first it would be my favourite (but it didn't so it isn't). It's worth getting into the whole Metal Gear universe before playing this one so that you appreciate all the little cross-references, but it could be interesting to do it the other way round (I wouldn't know).
Just finally I can't stress enough how much you should not play the GameCube version of MGS1. Imagine taking Ocarina of Time, updating the gameplay so that all the puzzles and level design are ruined because you now have vastly improved abilities thereby rendering the challenge null, and then imagine that they made all the cut-scenes ridiculous by adding ludicrous action-film sequences (such as Snake jumping off a missile Just Cause 2 adverts style - completely breaking any immersion or realism that they game sort of tries to convey elsewhere) and you've got what happened to MGS on teh GC. Don't do it. Please.